This invention relates to an additive combination and to various compositions comprising the same. More particularly, this invention relates to a combination oxidation inhibitor and to various organic compositions comprising the same.
As is well known, most organic compounds, especially those comprising relatively long hydrocarbon segments, are subject to autoxidation when the same are contacted with oxygen. This oxidation, in turn, results in a deterioration of the base material leading, generally, to increases in acid number and in sludge formation. In this regard, it is generally believed that the autoxidation first results in the formation of hydroperoxides which, in turn, decay, decompose or dissociate to yield corresponding acid, aldehydes, water and other undesirable oxygen-containing products and free radicals, which, in turn, may react to yield more hydroperoxides or higher molecular weight materials, which may or may not contain oxygen in some form. It will, of course, be appreciated that the acids thus formed will increase the total acid number while the higher molecular weight products will lead to increased viscosities and, indeed, if the same are insoluble in the initial media could result in sludge formation.
Heretofore, several materials have been proposed for use designed to prevent autoxidation. Generally, these materials will fall into one of three categories; viz., selective reducing agents, peroxide removers or decomposers and free radical scavengers. The first of these will, of course, be selectively oxidized, when oxygen is present, thereby preventing the formation of the undesirable hydroperoxide in the first place. The second of these, on the other hand, will complex with or decompose the peroxide immediately upon formation, generally, to a product which will not produce additional free radicals. Finally, the latter group simply converts the free radicals to an inert product. Of these, the materials generally classified as peroxide removers or decomposers and those classified as scavengers are most commonly used.
As is also well known, several materials qualify as peroxide removers or decomposers and these materials may be acidic, basic or neutral. Moreover, peroxide removers or decomposers from each of these classes have been used as oxidation inhibitors in various organic compositions such as lubricants, fuels and the like. The basic and neutral materials are, however, most generally used, especially in lubricant type compositions, since increased acid content is generally undesirable. Of the basic and neutral materials, the amines and particularly the naphthenic and aliphatic amines, are commonly used and such use is well known in the prior art. Moreover, amines as well as other possible peroxide complexing agents or decomposers are used for other purposes in organic compositions such as lubricating and specialty oils and fuels.
Similarly, several materials are well-known free radical scavengers and the use of a large number of such scavengers in organic compositions has, heretofore, been proposed. These materials function by interrupting the chain reaction by which oxidation takes place. This chain reaction proceeds by a two-step process. In the first reaction, a peroxy radical or an alkyoxy radical formed from peroxide decomposition attacks the material being oxidized to abstract a hydrogen atom by breaking a carbon hydrogen bond. This results in the formation of a peroxide or an alcohol and an alkyl type radical from the substrate. In the second reaction the alkyl type radical combines with oxygen to form a peroxy radical which can react to start off the chain again. The second reaction is extremely rapid compared to the first. As a result, the concentration of peroxy radicals in normally several magnitudes greater than that of the hydrocarbon radicals. The free radical scavengers normally employed for inhibition are compounds which can react very rapidly with the peroxy radicals to destroy the chain.
Transition metals are well known catalysts for autoxidation. It is well established in the prior art that when organic materials such as lubricating oils, polymers or -plastics must be used in contact with iron, copper or other transition metals it is much more difficult to stabilize them against oxidative degradation. Many laboratory tests for oxidation stability incorporate transition metals, either in their metallic form or as low concentrations of soluble compounds, to accelerate the test and to provide results which are more representative of degradation under conditions of expected use. As the concentration of a transition metal increases, the catalytic activity increases until, at a concentration often referred to as the critical concentration, the catalytic activity drops sharply to zero and at concentrations above this level the transition metal salts function as inhibitors. This phenomenon too is known in the prior art and has been discussed extensively, either directly or indirectly, in a large number of papers published throughout the world but the reasons for this abrupt change have never been completely explained.
Notwithstanding the general knowledge with respect to the ability of the transition metal salts to function as inhibitors, the use of such materials as inhibitors in organic compositions subject to autoxidation has been limited. Reasons for this are that the transition from catalysts to inhibitor has usually been observed to occur at relatively high concentrations; second, that the reasons why an abrupt change from catalyst to inhibitor takes place has not been understood and third, it has been impossible to predict the critical concentration level. Prior attempts to use transition metals as inhibitors have, therefore, produced inconsistent results. This limited use has continued notwithstanding that the cost of the transition metal salts is often less than the cost of the more conventional oxidation inhibitors and notwithstanding evidence that the same will often function as oxidation inhibitors yielding results superior to those often obtained with the more conventional inhibitors.